How to include a custom field in the woocommerce email?

Adding custom fields to emails

/* To use: 
   1. Add this snippet to your theme's functions.php file
   2. Change the meta key names in the snippet
   3. Create a custom field in the order post - e.g. key = "Tracking Code" value = abcdefg
   4. When next updating the status, or during any other event which emails 
          the user, they will see this field in their email
 */

add_filter('woocommerce_email_order_meta_keys', 'my_custom_order_meta_keys');
function my_custom_order_meta_keys( $keys ) {
  $keys[] = 'Tracking Code'; // This will look for a custom field called 'Tracking Code' and add it to emails
  return $keys;
}

Woocommerce documentation: Customizing checkout fields using actions and filters